My invention relates to welding processes and particularly for production of welds without residual stresses or with residual stresses of low value and reduced porosity. The process described, however, can be applied to any operation such as casting and brazing. The process is also useful for reducing the residual stresses when molten metal is solidified or when the product which can be a casting, forging or a weldment is heated to temperatures at which dislocation movements can be achieved with the application of low stress cycles.
It is known that stresses come into existance around each dislocation. Dislocation, a known natural metallurgical defect, exists in every metal crystal and it is not possible to eliminate dislocations entirely from any metal. If the density of dislocations can be reduced however, the gross stress which remains in the metal as residual stress can be reduced. It is advantageous to produce an end product without harmful levels of residual stresses. At higher temperatures, such as those near the solidification temperature of a metal, lower external stresses can cause greater movements of dislocations. Mechanisms such as cross-slip and annihilation are known to come into effect in crystals, reducing the density of dislocations. Reduction in their density can thus be brought about. At any other lower temperatures higher external stresses may be required for the movements of dislocations.
Thus a dislocation movements at lower temperature, i.e. room temperature at 20.degree. Centigrade, are known to require external stresses of higher value for their movements in comparison to stresses close to solidification temperature for, e.g., aluminum alloys. Dislocation density in various metals is known to be based on variables, some of which are temperature dependent. In some metals, such as pure aluminum, non-temperature dependent factors do not exist. Accordingly, the application of vibratory stress at room temperature after the weld is made has resulted in little success.
Another method used in industry for reduction of residual stresses involves the process of annealing. Annealing depends upon the movement of dislocations at annealing temperatures. This process is based on the thermal property of the metal at its annealing temperature. No precise time of holding at annealing temperature can be calculated, since data regarding the residual stresses of a metal product is generally unknown due to its complex, inter-crystalline nature. Thus, the time of holding a metal part at the annealing temperature usually depends on the thickness of the metal involved. Dislocation reducing mechanisms come into effect during the annealing process, and a new balance of residual stresses at low values comes into existance. In contrast, at absolute zero temperature movement of dislocations must occur by athermal means.
Porosity is another defect in welds. And only by use of inert atmosphere, fluxes, and materials which are known to give least gaseous by-products; can some control be exerted on formation of this defect. Once a weld solidifies, its porosity cannot be reduced. It is, however, possible to reduce the porosity in welds, if stress cycles are applied throughout the welding operation so that during solidification, semi-solid and liquid stages, entrapped gases are driven out.
During semi-solid stage of the weld metal, it is known that metal atoms deposit on solidified atomic planes which are available, thereby facilitating the growth of crystals. There are numerous orientations in which further growth due the deposit of atoms can take place; an advancing solidification wavefront generates numerous dislocations. A small force applied at this instant can reduce the density of dislocations.
It is known that residual stresses in welded materials can bring about distortion, and may become detrimental to the structure under certain conditions.
Since additional outlay is needed for annealing industrial furnaces, handling and fuel costs, there is a need for a process which will eliminate such costs and time delays in stress relieving operations of weldments. There is also a need for a controllable process which will reduce the entrapment of gases in welds during welding, since welds without porosity are desirable. The instant invention is directed toward those needs.